I don't think there is any size limits or regulations in Arizona for bicycle trailers. But a trailer 4 ft wide would be dangerous driving on the side of the road. It would require cars to get in the other lane to pass if another lane was available. I have had great success with B.O.B. trailers. They track true and are made of good materials.
Thank you for your quick replies. My BAD! I meant more like 21 inches wide. I want to put two solar panels that I got in a deal on top of a trailer in which I will haul prospecting equipment as well as some batteries. The panels are 21 inches wide and 4 foot long. So they will go on top of the trailer making it about 21 inches wide by 8 foot long. It would give me access to areas that are now not accessible except by foot and carrying stuff just doesn't cut it. If there are no size limits then I'm good to go. Thank you again.